Microelectromechanical systems (“MEMS”) are used in a growing number of applications. For example, MEMS currently are implemented as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have a structure suspended above a substrate, and associated on-chip or off-chip electronics that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).
The associated electronics, substrate, and movable structure typically are formed on one or more dies (referred to herein simply as a “die”) that often are secured within a package. For example, the package, which typically hermetically seals the die when implemented as an accelerometer, may be produced from ceramic or plastic. The package includes interconnects that permit the electronics to transmit the movement data to the external devices. To secure the die to the package interior, the bottom surface of the die commonly is bonded (e.g., with an adhesive or solder) to an internal surface (e.g., a die attach pad) of the package. Accordingly, substantially all of the area of the bottom die surface is bonded to the internal surface the package.
Problems can arise, however, when the temperatures of the two surfaces change. In particular, because both surfaces can have different coefficients of thermal expansion, the package can apply a mechanical stress to the substrate of the die. This stress (referred to in the art as “linear stress,” which, in this case, is thermally induced) undesirably can bend or flex the substrate to an unknown curvature. Substrate bending or flexing consequently can affect movement of the die structures, thus causing the output data representing the property being measured (e.g., acceleration) to be erroneous. In a similar manner, mechanically induced linear or torsional stress applied to the package also can be translated to the die, thus causing the same undesirable effects.